The present invention relates to flexible coaxial cables and more particularly to such cables of small section suitable for use in particular in medical imaging, or in portable transmission equipment such as computers, or for conveying microwaves.
One of the requirements that needs to be satisfied is that the outer conductor or shield of such a cable must remain fully effective even though the cable, and thus the shield, are flexible.
Document GB-A-2 130 430 discloses a flexible coaxial cable which has a flexible conductive core, flexible insulation around said core, a conductive shield that is likewise flexible around said insulation, and an optional flexible outer protective sheath. The shield of the cable is constituted by an inner metal tape which is folded longitudinally around the insulation, an outer metal tape which is wound helically around the inner tape, and an optional metal braid, which is then mounted around the outer tape and is designed to enable connections to be made to the shield at the ends of the cable. The edges of the inner tape overlap and are bonded together along the cable, and the edges of the outer tape overlap widely from one turn to the next. Each of the tapes can be made of a single metal or can be made of a composite material, e.g. tinned copper or a plastic/metal laminate.
In that known cable, the outer tape serves to cover any cracks or breaks that might appear in the inner tape under the effect of the cable being bent/straightened. Nevertheless, the turns themselves tend to move during such bending/straightening of the cable, thereby changing the contact resistance and the assembly configuration of the turns relative to one another and relative to the inner tape, leading to current flowing along a helical path in the outer tape and thus limiting the performance of the shield.
Document EP-A-0 236 096 also discloses a flexible coaxial cable which avoids the drawbacks of the cable known from the above-mentioned document, and to this end it has a shield comprising a copper tape placed longitudinally or wound helically around the insulation and having overlapping edges, a copper braid placed around the tape, and a layer of metal closing the opening along the overlapping edges of said tape and the interstices in the braid and securing the braid to said tape, such that the shield is flexible and without openings.
To manufacture such a cable, after the shield and the braid have been put into place, the cable is passed through a bath of molten metal, specifically solder. While the molten metal is being applied, the tape acts as a thermal shield so as to isolate the insulation thermally from the molten metal, given that in the absence of the tape, the molten metal would come directly into contact with the insulation and could harm it.
The coaxial cable obtained in that way is flexible while also having a shield that provides high performance. Nevertheless, the layer of metal between the braid and the tape is not of uniform thickness all around the cable, because of the fluidity of the metal in the molten state. In addition, the method of manufacturing such a shield is lengthy and not very convenient. As mentioned in that document, it cannot be performed continuously in line because of the very different speeds used in the different stages of the method. Braid installation takes place much more slowly than do the other stages. Furthermore, although using a tight braid is advantageous in order to enable its interstices to be properly closed by the molten metal, it makes it more difficult and less reliable for the molten metal to flow through the braid so as to bond it to the tape and also close the line of opening at the edges of the tape. Furthermore, the use of a bath of molten solder requires safety devices to avoid the risk of burning and of inhaling vapor.
An object of the present invention is to provide a flexible coaxial cable having a shield of reliable structure leading to performance that is excellent and stable over time, and suitable for being manufactured by means of a method that is simple and fast.
The present invention provides a flexible coaxial cable comprising a flexible conductive core, flexible insulation around said core, and a flexible metal shield around said insulation, said shield comprising a first metal tape provided with a metal coating on at least one face thereof and wound helically around said insulation so as to present successive turns that overlap in a helical overlap zone that is continuous along said cable, wherein said first tape is closed in leakproof manner onto itself throughout said overlap zone between said turns by said coating bonding said turns to one another.
Likewise, the invention also provides a flexible coaxial cable comprising a flexible conductive core, flexible insulation around said core, and a flexible metal shield around said insulation, said shield comprising a first metal tape and an outer second metal tape, both tapes being wound helically around said insulation, and at least one of said tapes being provided with a metal coating on at least one of its faces, wherein each of said tapes presents turns leaving a respective helical gap that is continuous along said cable, wherein said turns of said second tape completely overlap the gap in the first tape and partially overlap the turns of said first tape, and wherein the overlap zones between the turns of said tapes are bonded together in leakproof manner by said coating provided on at least one of the two facing faces of said tape.
Advantageously, the cable presents at least one of the following additional characteristics:
said first tape has a winding pitch lying in the range 0.2 millimeters (mm) to 10 mm and said cable has a diameter across said insulation lying in the range 0.2 mm to 8 mm; and
said shield includes an inner metal tape placed longitudinally directly around said insulation, and having edges that overlap.
The invention also provides a method of manufacturing said cable, the method consisting in subjecting the cable provided with said shield to peripheral heating to bond together the turns of each tape constituting said shield, without external bonding material being supplied.
Advantageously, the peripheral heating for bonding is provided by means of an induction coil powered at high frequency and provided on a manufacturing line for making said shield continuously.